1. Field of Invention
The present invention relates to a method for transferring graphene nondestructively and at low cost, and more particularly to a low cost and damage-free transferring method which utilizes the moving action and the intercalation effect of gas bubbles produced by an electrolysis process to transfer graphene from an initial substrate to a non-selective target substrate. The method is suitable for transferring monolayer or few-layer graphene on the surface of conductor or semi-conductor substrate without damage and at low cost.
2. Description of Related Arts
Graphene has a two-dimensional honeycomb crystal structure formed by a single-layer of densely packed carbon atoms, which is the basic structural unit for the construction of other digital carbon materials (such as zero-dimensional fullerenes, one-dimensional carbon nanotube and three-dimensional graphite). The unique crystal structure of graphene provides excellent electrical, thermal and mechanical properties, for examples, the electron mobility is up to 200,000 cm2/V·s and the thermal conductivity is up to 5300 W/m·k. It is possible for the graphene to be widely used in the multi-functional nanoelectronic devices, transparent conductive film, composite materials, catalytic materials, energy storage materials, field emission materials, gas sensor, gas storage materials and other fields. In order to utilize the numerous excellent characteristics of graphene, high quality graphene preparation and graphene transfer to a particular substrate becomes the important factors. Since stable graphene is first isolated by tape exfoliation method (or the micro-mechanical exfoliation method) by the research study group of the University of Manchester in 2004, many graphene preparation methods are then developed, which includes chemical exfoliation method, epitaxial growth method and chemical vapor deposition (CVD) method. In view of the relatively simple preparation process and relatively large scale of production, the graphene which is prepared by chemical exfoliation has already been widely used in composite materials, flexible transparent conductive film and energy storage electrode materials. However, because the quality of graphene obtained by chemical exfoliation is relatively poor and numerous structural defects are existed, the structural properties such as the size and the number of layer of graphene are difficult to control. The CVD and the epitaxial growth methods are the major method of preparing graphene in high quality. Through the control of preparation parameters which includes temperature, carbon source and pressure of the process, highly crystalline graphene can be grown onto many different substrate surface (metal and non-metal) and the size and number of layer of graphene can be controlled within a certain range. For graphene studies in relation to its characteristics, physical measurement and application research, the graphene is generally required to be placed on a particular substrate which is different from the substrate on which the graphene is grown. Therefore, development of graphene transfer technology for high quality graphene has very important role and significance on promoting the research of graphene materials.
At present, the two major types of graphene transfer technologies are chemical etching of substrate and direct transfer method. In view of the graphene at atomic level or with a thickness of a few nanometers, its macroscopic strength is very low and is very susceptible to damage, therefore its isolation or separation from its initial substrate without damages is the major challenge for graphene transfer. For graphene grown on surface of transition metal by CVD or epitaxial growth method, method of etching substrate can be used to solve the problem. However, because the etching method sacrifices the metal substrate, the metal materials of the substrate is consumed during the transfer process and therefore the cost of graphene preparation is increased significantly (especially for high value substrate). In addition, the steps are complicated, the process cycle is long and serious environmental pollution is caused. The method is also not suitable for transferring graphene on precious metal substrate such as rubidium (Ru) and platinum (Pt) which is highly chemically stable. For graphene grown on high value substrate, direct transfer method can be used, which is utilizing a transfer medium with stronger binding force (such as tape, adhesive and etc.) with graphene to peel off graphene directly from the substrate surface. This method does not consume any substrate materials, or utilizing chemical reagent which is corrosive or polluting. However, this method can cause damage to graphene easily and fails to realize a damage-free transfer for high quality graphene. In summary, a damage-free graphene transfer technology (damage-free for substrate materials and fro graphene) is urgently needed, which, in some extent, is critical to future development prospects of high quality graphene.